1. Field of the Invention
This is a continuation of U.S. application Ser. No. 07/340,457 filed Apr. 19, 1989 now abandoned.
The present invention relates to a thermal transfer film used in thermal recording devices such as thermal printer, etc. and more particularly to a thermal transfer film which can prevent static charging phenomenon on recording, sticking phenomenon and staining of a thermal head.
2. Discussion on Related Arts
Recently, thermal transfer films comprising a base film coated with a heat-meltable ink have come to be used rapidly in thermal recording devices such as thermal printers and thermal facsimiles. Thus clear images are obtained on plain papers. That is, a plain paper and a heat-meltable ink layer of a thermal transfer film are brought into close contact with each other and are subjected to localized heating by a pulse signal from a thermal head opposite to the heat-meltable ink layer.
The heated heat-meltable ink layer is molten and transferred to the plain paper to give an image.
As a base film of such thermal transfer films, there have been known various films such as polyester, polycarbonate, polystyrene, polyethylene, polypropylene, vinyl chloride, vinylidene chloride, polyimide and polyamide.
In many cases, a heat resistant layer of a silicone resin and the like is provided on the surface which contacts with a thermal head to prevent sticking.
However, thermal transfer films with the above-exemplified base films are high in surface resistivity whose surface contacts with a thermal head, namely, at least 10.sup.14 and hence have the defect that static electricity is often generated at thermal transfer printing.
Static electricity is generated due to friction between the thermal head and the thermal transfer film on thermal transfer printing and when the thermal transfer film is peeled from a plain paper.
Obstruction caused by occurrence of static electricity is an electric shock given to human body at the time of changing of used thermal transfer film roll charged with 10 KV or higher. Besides, dust collected on thermal head sometimes result in unclear images. Further, the plain paper is also charged to deteriorate travelling property of the paper in some cases.
Hitherto, various proposals have made to improve this static charging phenomenon. For example, Japanese Patent Kokai (Laid-Open) No. 129789/82 has proposed a method of providing a resin layer containing a surface active agent or an organic salt on the surface of a base film opposite to the ink layer. However, the surface active agent and the organic salt are still insufficient in antistatic effect (the effect of preventing static charging phenomenon) because these are contained in a resin layer. That is, in order that a surface active agent exhibits the effect as antistatic agent, the surface active agent needs to be migrated to the surface of a resin layer and is present on the surface in such a state that oleophilic portion of the molecule of the surface active agent faces inner portion of the resin and hydrophilic portion faces air and thus water in air is adsorbed to the hydrophilic portion to exhibit the antistatic effect. If compatibility between the resin and the surface active agent is high, less migration to the surface of the resin occurs and the effect is difficult to be exhibited.
On the other hand, if the compatibility is poor, the surface active agent moves to the surface of resin to exhibit antistatic effect, but there is the defect of the surface active agent, that is staining a thermal head.
Japanese Patent Kokai (Laid-Open) No. 151095/85 proposes thermal transfer films containing an electrically conductive material and discloses that this has antistatic effect. Specifically, this patent publication proposes a thermal transfer film where a conductive material is provided as a layer on a support opposite to a heat meltable ink layer, a thermal transfer film where the conductive material is provided as a layer between the support and the heat meltable ink layer, a thermal transfer film where the conductive material is contained in the heat meltable ink layer and a thermal transfer film where the conductive layer is contained in a heat resisting layer. As the electrically conductive material, there are mentioned NaCl, KCl, MgCI.sub.2, anionic surface active agents, cationic surface active agents, nonionic surface active agents ampholitic surfactant, Al, Cu, Zn, carbon, polyelectrolytes, organic semiconductors and the like. In order to provide these conductive materials in the form of a layer, they must be contained in resins and those which contain NaCl, KCl, MgCl.sub.2, or surface active agents are still insufficient in antistatic effect.
Furthermore, resins containing Al, Cu, Zn or carbon have antistatic effect, but base film of thermal trahsfer film becomes opaque and weight of coated heat meltable ink cannot be controlled by transmission density. Thus, there remain practical problems.